The present invention relates to a unit for heating a fluid and a plant using such a unit as a part thereof.
Units and complete systems based thereon for heating fluids are known in a great number of different forms. In many private houses and in industry, instantaneous water heaters, run for example on gas or electricity, are in use, in which the desired temperature of the heated fluid is controlled by turning on and off the heating fluid or energy carrier or by changing its rate of flow. In less frequent cases heat is produced in fluid heating apparatus by internal friction, for example in a liquid, or eddy currents are used, that are produced in a conductor when it is moved through a magnetic field.
As a rule mechanical energy is not turned directly into heat energy, because the conversion of primary energy, for example in the form of the force of the wind or of water, into electrical energy has made it possible for the energy to be put to many more different purposes and furthermore, it is then simpler for the energy to be supplied to the point at which it is needed. This being so, it is hardly surprising that for heating purposes nearly the only forms of apparatus used are those run on oil, gas or electricity. For some special purposes, however, such heating apparatus as are presently widely used have great shortcomings.
Whereas under steady state conditions it is relatively simple to keep an adjustment to a given temperature, known automatic control and heating systems designed so far have such a slow response under dynamic or changing conditions that is very hard to keep the temperature of the heated fluid within a narrow range without the output temperature changing every time there is a change in the input temperature and without being dependent on its flow rate, this being, more specially, because of a certain lag in the heating effect, caused by stored heat, at the points at which heat is given up by the heat producing or supply parts to the fluid to be heated. It is for this reason that so far automatic control with the quickest possible response time has been produced by using instantaneous heaters in which the heating coil element is placed right in the flowing fluid to be heated. The use of such apparatus is however limited to liquids with a high electrical resistance and to those that are not flammable.